From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID autolearn=no autolearn_force=no version=3.4.4 X-Google-Language: ENGLISH,ASCII-7-bit X-Google-Thread: 103376,c7cbae112ac5e4c3 X-Google-Attributes: gid103376,public From: Thomas Handler Subject: Re: Tasking and preemption Date: 1998/12/13 Message-ID: <3672FB41.53313983@umundum.vol.at>#1/1 X-Deja-AN: 421997660 Content-Transfer-Encoding: 7bit References: <36714AA1.96BD1AC5@systems.at> <74rr7n$4sp$1@nnrp1.dejanews.com> Content-Type: text/plain; charset=us-ascii Organization: Toemmsn's Homeoffice Mime-Version: 1.0 Newsgroups: comp.lang.ada Date: 1998-12-13T00:00:00+00:00 List-Id: dennison@telepath.com wrote: > > In article <36714AA1.96BD1AC5@systems.at>, > Thomas Handler wrote: > > Hi to everyone! > > > > I'm trying to get into Ada and have now passed the tasking mechanism > > provided by Ada95. > > My problem is that my understanding of preempting seems not to match > > with that of my system ;-) > > > > I'm using GNAT 3.09p on Linux with libpthreads and when doing a simple > > program with two tasks where each task body is simply puttnig a string > > in a loop to the screen the tasks seem to have a real cooperative > > behavior, i.e. Task B starts working after Task A has finished > > (independent of the length of the loop). > > Building a delay into the loop has the desired effect. > > Feel fortunate its working that way. Text_IO is typically not task-safe. You > shouldn't be trying to write to the same file (the screen counts as a file) > from two different concurrently-running tasks. OK, it was not a quite good example using Text_IO but it was just for a test ;-) > > > So my question is: In the RM preemption is based on calling blocking > > statements (like delay), as far as I understand there is no definition > > of thread supported preemption. Is this true (i.e. I have to take care > > in my program to make calls to blocking statements) or am I terribly > > wrong? > > There are multitudes of different scheduling policies that your system could > be using. The only predfined one is FIFO_Within_Priorities (D.2.2). Under > this policy a running task could concieveably hog the CPU forever unless it > performs a rendezvous, accept, or a delay. Check out Annex D of the LRM. It > has a very thourough treatment of this subject. I have read this annex several times in LRM and AARM and with your above answer it seems that I got itz the right way. So this does actually mean that having a task that does heavy computations has to be written a.) in a way that ensures that a blocking statement will be executed from time to time b.) this task runs on lower priority and the other tasks execute a blocking statement (will this work ?) c.) use a different (implementation dependant) scheduling policy (BTW: which task dispatching policies are defined for GNAT?) I have to admit that I have not used threads ever before on Unix due to the fact that I have gained some experiences with them on a propietary system developed by my former employer. After a look on the PThread documentation I decided not to use them with C/C++ since the latter of the combination itself is already troublesome :-) But it seems strange to me since my understanding of threads always was that they are preempting (that's why you a thread-safe library will help a lot ;-) and GNAT uses libpthread. So I'm a bit surprised at the moment. Ciao, Thomas Handler > > -- > T.E.D. > > -----------== Posted via Deja News, The Discussion Network ==---------- > http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own